49 CFR §179.220-8

1. (a)Tank heads of the inner container, inner container compartments and outer shell must be of approved contour, and may be flanged and dished or ellipsoidal for pressure on concave side.
2. (b)Flanged and dished heads must have main inside radius not exceeding 10 feet and inside knuckle radius must be not less than 3 3/4 inches for steel and alloy steel tanks nor less than 5 inches for aluminum alloy tanks.
3. (c)Ellipsoidal heads must be an ellipsoid of revolution in which the major axis must equal the diameter of the shell and the minor axis must be one-half the major axis.